Solar flares set the Sun quaking

Left panel: Smoothed frequency-time diagram of global oscillations of the Sun as measured by
the VIRGO instrument on SOHO. The color coding represents the strength of the oscillations.
The bright horizontal lines are strongly correlated with solar flares, represented
by the solar X-ray flux in the right panel. These lines are most prominent in the high-frequency region
of the spectrum. The vertical line at 5.55 mHz in the left panel is an instrumental artifact.

Data from SOHO's VIRGO instrument show clearly that powerful starquakes ripple around the Sun in the wake
of the mighty solar flares that explode above its surface. The observations give solar physicists a new
insight into a long running solar mystery, and may even provide a way of studying other stars, too.

The outermost quarter of the Sun's interior is a constantly churning maelstrom of hot gas.
Turbulence in this region causes ripples that criss-cross the solar surface, making it heave up and
down in a patchwork pattern of peaks and troughs.

SOHO has proved to be an exceptional spacecraft for studying this phenomenon. Discovering how the
ripples move around the Sun has provided valuable
information about the Sun's interior conditions. A class of oscillations called the 5-minute
oscillations with a frequency of around 3 millihertz have proven particularly useful.

According to conventional thinking, the 5-minute oscillations can be thought of as the sound you
would get from a bell sitting in the middle of the desert and constantly being touched by random
sand grains, blown on the wind. What Christopher Karoff and Hans Kjeldsen, both at the University of
Århus, Denmark, saw in the data, however, was very
different. "The signal we saw was like someone occasionally walking up to the bell and striking
it, which told us that there was something missing from our understanding of how the Sun works," Karoff
says.

So they began looking for the culprit and discovered an unexpected correlation with solar flares.
It seemed that when the number of solar flares went up, so did the strength of the the 5-minute
oscillations. "The strength of the correlation was so strong that there can be no doubt about it,"
says Karoff.
A similar phenomenon is known on Earth in the aftermath of large earthquakes. For example, after
the 2004 Sumatra-Andaman Earthquake, the whole Earth rang with seismic waves like a vibrating bell
for several weeks.

The correlation is not the end of the story. Now the researchers have to work to understand the
mechanism by which the flares cause the oscillations. "We are not completely sure how the solar
flares excite the global oscillations," says Karoff.
In a broader context the correlation suggests that, by looking for similar oscillations within other
stars, astronomers can monitor them for flares. Already, Karoff has
used high-technology instruments at major ground-based telescopes to look at other Sun-like
stars. In several cases, he detected the tell-tale signs of oscillations that might originate from
flares.

"Now we need to monitor these stars for hundreds of days," he says. That will require dedicated
spacecraft, such as the recently launched COROT mission. The hard work, it seems, is just starting.